Certain spine conditions, defects, deformities (e.g., scoliosis) as well as injuries may lead to structural instabilities, nerve or spinal cord damage, pain or other manifestations. Back pain (e.g., pain associated with the spinal column or mechanical back pain) may be caused by structural defects, by injuries or over the course of time from the aging process. For example, back pain is frequently caused by repetitive and/or high stress loads on or increased motion around certain boney or soft tissue structures. The natural course of aging leads to degeneration of the disc, loss of disc height, and instability of the spine among other structural manifestations at or around the spine. With disc degeneration, the posterior elements of the spine bear increased loads with disc height loss, and subsequently attempt to compensate with the formation of osteophytes and thickening of various stabilizing spinal ligaments. The facet joints may develop pain due to arthritic changes caused by increased loads. Furthermore, osteophytes in the neural foramina and thickening of spinal ligaments can lead to spinal stenosis, or impingement of nerve roots in the spinal canal or neural foramina. Scoliosis may also create disproportionate loading on various elements of the spine and may require correction, stabilization or fusion.
Pain caused by abnormal motion of the spine has long been treated by fixation of the motion segment. Spinal fusion is one way of stabilizing the spine to reduce pain. In general, it is believed that anterior interbody or posterior fusion prevents movement between one or more joints where pain is occurring from irritating motion. Fusion typically involves removal of the native disc, packing bone graft material into the resulting intervertebral space, and anterior stabilization, e.g., with intervertebral fusion cages or posterior stabilization, e.g., supporting the spinal column with internal fixation devices such as rods and screws. Internal fixation is typically an adjunct to attain intervertebral fusion. Many types of spine implants are available for performing spinal fixation, including the Harrington hook and rod, pedicle screws and rods, interbody fusion cages, and sublaminar wires.
Spinal stenosis pain or from impingement of nerve roots in the neural foramina has been treated by laminectomy and foraminotomy, and sometimes reinforced with rod and screw fixation of the posterior spine. More recently, surgeons have attempted to relieve spinal stenosis by distracting adjacent spinous processes with a wedge implant. Pain due to instability of the spine has also been treated with dynamic stabilization of the posterior spine, using elastic bands that connect pedicles of adjacent vertebrae.
The typical techniques for fusion, distraction, decompression, and dynamic stabilization require open surgical procedures with removal of stabilizing muscles from the spinal column, leading to pain, blood loss, and prolonged recovery periods after surgery due in part to the disruption of associated body structures or tissue during the procedures.
To reduce the invasiveness of fusion procedures, some methods of fusion have been proposed that do not require the extensive stripping of muscles away from the spinal column of earlier approaches. These involve posteriorly or laterally accessing the spine and creating spaces adjacent the spine for posterior stabilization. Some of these procedures include fusion via small working channels, created with dilator type devices or an external guide to create a trajectory channel between two ipsilateral neighboring pedicle screws. Also, placing support structures between adjacent pedicle screws and across a joint requires accessing and working in an area from a difficult angle (the support structure is typically oriented somewhat perpendicular to an angle of access and through muscle and connective tissue). Furthermore, these stabilization devices typically involve the use of 4 pedicle screws (each having a risk associated with it when placed in the spine), two on each side of a motion segment, and are not ideally suited for percutaneous stabilization required across more than one or two segments. Accordingly, it would be desirable to provide a less invasive or less disruptive segmental spine stabilization procedure and implant that has a reduced risk of damage or injury to associated tissue. It would also be desirable to provide an implanted posterior spine system that may be used to stabilize more than two motion segments in a less disruptive or less invasive manner.
One method of fusing a vertebra has been proposed using bilateral screws through the lamina using a posterior approach. However, geometric placement of the device is difficult and the procedure is considered dangerous because the laminar screws could enter through anteriorly into the spinal canal and cause nerve damage.
Accordingly, it would be desirable to provide a device that reduces the difficulties risks of the current procedures. It would also be desirable to provide a device that can be placed in a less disruptive or less invasive manner than commonly used procedures.
Unintended consequences of fixation include stress shielding of bone, as well as transfer of load to adjacent, still dynamic motion segments, and eventual degeneration of adjacent motion segments. Flexible stabilization of motion segments with plastic, rubber, super-elastic metals, fabric, and other elastic materials has been proposed to provide a degree of dynamic stabilization of some joints. Many of these constructs are not load bearing. Dynamic stabilization from pedicle screw to pedicle screw along the length of the spine has been proposed. However, this device has the disadvantage of requiring placement of 4 pedicle screws and associated tissue disruption.
Due to the risks, inconvenience, and recovery time required for surgical implantation of spinal devices, some patients may continue to prefer rigid fixation of a painful or degenerative motion segment over dynamic stabilization of the joint. In addition, doctors may be reluctant to recommend dynamic stabilization for patients with back pain, because it may not alleviate pain to a patient's satisfaction.
Furthermore, even in patients who experience good relief of pain with dynamic stabilizers, it is anticipated that while the onset of arthritic changes may be deferred, many patients will still eventually proceed to develop degeneration, and require fixation of the motion segment to obtain pain relief. Repeat spine procedures to remove one implant and replace it with another are associated with complications related to bleeding, surgical adhesions, destruction of bone, and other generic risks associated with surgical procedures. Accordingly, improved devices that address these issues would be desirable.
A number of spinal deformities exist where the spine is abnormally twisted and or curved. Scoliosis is typically considered an abnormal lateral curvature of the vertebral column.
Correction of scoliosis has been attempted a number of ways. Typically correction is followed by fusion. A Harrington rod has been used where a compressing or distracting rod is attached above and below a curved arch of the deformity. The spine is stretched longitudinally to straighten the spine as the rod is lengthened. The spine is then fused. The correction force in this device and in similar devices is a distraction force that may have several drawbacks including possible spinal cord damage, as well as the high loading on the upper and lower attachment sites. Nowadays, segmental hook and screw fixation exists for distraction and derotation corrective forces.
A Luque device has been used where the spine is wired to a rod at multiple fixation points along the rod and pulls the spine to the rod. The spine is pulled to the rod with a wire and the spine is then fused. This does not provide significant adjustment over time and requires fusion. Once completed this does not provide an opportunity for delayed adjustment over time. Anterior procedures also exist in the form of fusion and newer technology involving staples across the disc space that obviate the need for fusion but still correct the deformity. The corrective force is derotation with or without compression.
Accordingly it would be desirable to provide an improved corrective device for treating scoliosis or other deformities. It would also be desirable to provide a device that may be used without fusion.
Spine surgeons commonly use metallic or polymeric implants to effect or augment the biomechanics of the spine. The implants frequently are attached or anchored to bone of the spine. Sites typically considered appropriate for boney attachment have high density or surface area, such as, for example, the pedicle bone, the vertebral body or the cortical bone of the lamina. The spinous process contains thin walls of cortical bone, and thus, has been considered as not ideal for anchoring spinal implants as they may not support the implants under physiologic loads, or the intermittent high loads seen in traumatic situations. Fixation has been attempted from spinous process to spinous process with poor results.
A translaminar facet screw as used by some surgeons goes through the base of spinous process to access the cancellous bone of the lamina. A disadvantage of this device is that it is not suitable for attaching to a pedicle screw and the depth and angle during deployment can be very difficult to track or visualize, thus increasing the possibility that the screw would extend into the spinal canal. A facet screw is screwed between opposing facets of a zygapophyseal joint.